1. Field of the Invention
The present invention relates in general to a novel method of designing an ophthalmic lens, and an ophthalmic lens produced by the method. More particularly, the present invention is concerned with a method of determining specifications of the ophthalmic lens, which ophthalmic lens assures a high degree of visual acuity with high stability while avoiding a variation of the visual acuity even when the optical axis of the ophthalmic lens is offset from the optical axis of the eye of the lens wearer due to a movement of the lens on the cornea or in the eye. Further, the present invention is concerned with a method of producing the ophthalmic lens based on the determined specifications. The present invention also relates to an ophthalmic lens which is produced by the method.
2. Discussion of Related Art
Conventionally, an ophthalmic lens such as a contact lens and an intraocular lens is designed independently of the eye to which the ophthalmic lens is applied. In other words, the ophthalmic lens is produced so as to assure an optimum characteristic of imaging on its optical axis. For attaining the optimum imaging characteristic, various techniques are proposed. For instance, Japanese Patent No. 2859092 discloses a method of designing an ophthalmic lens, wherein a path of light which is incident on each reference point on one of the opposite lens surfaces that has been already formed into an intended configuration is taken into account, and an angle of inclination of the other lens surface at a point located on the light path is determined so that the light undergoes an intended refracting force. Japanese Patent No. 2913191 discloses a method of designing a lens by a ray tracing technique in an attempt to reduce a spherical aberration of the lens. In the disclosed method, the configuration of the refracting surface of the lens is designed as follows. The inclination of a curved surface at an arbitrary height as measured from the optical axis of the refracting surface is determined such that the light passing the arbitrary height passes a desired final pass point which is set in advance, after the light has passed through the lens. Based on the successively obtained inclinations of the curved surface at different heights, the configuration of the curved surface as a whole is designed.
Japanese Patent No. 3022640 discloses a method of producing an aspheric lens, comprising the steps of: constructing a mathematical model of a system of a human eye and a preliminary lens; performing an analysis using the model to trace light ray paths through the lens-eye system; varying a value of the aspheric constant k for the preliminary lens to achieve a lens-eye system with a trace of light ray paths optimized for sharpest focus by minimizing retinal spot size of the rays.
The above-described conventional methods of designing or producing the ophthalmic lens aim at optimizing the imaging characteristic on the optical axis of the lens. These methods are for optimizing the imaging characteristic with respect to an optical schematic of the human eye (schematic eye) by taking into account the eye optical system. The ophthalmic lens designed or produced according to the conventional methods assures the optimum imaging characteristic on its optical axis. These methods, however, ignore the fact that the center of the pupil of the eye (the optical axis of the eye) is usually not aligned with the optical center of the lens while the lens is applied to or worn on the eye. Accordingly, the optical characteristic of the optical system consisting of the lens and the eye tends to be deteriorated when the lens is offset from the optical axis of the eye. For example, the optical center of the contact lens is not aligned with the center of the pupil of the eye in most instances when the contact lens is worn on the eye. The movement of the contact lens on the cornea of the lens wearer's eye and the stable position of the contact lens on the cornea in which the contact lens is stably held in place vary depending upon the individual lens wearers. Accordingly, the visual acuity and stability tend to fluctuate depending upon the movement and stable position of the contact lens on the cornea of the eye.